I have been building facades with coro for years.
It's the single best thing for backlighting.
I buy it by the pallet and cut it on my big old' panel wall saw.
I would however orient your panels vertically, as they will be much brighter. you sort of defeat the purpose of using the coro to pipe the light when you turn it on it's side.
Vertical is much brighter.
Also, I'd try doing a dado groove all the way up, and pocket drill those corners. Abandon the 45 degrees and go with 90 joints. they will be much stronger, and easier to square up.
I've been selling aluminum facade kits made for coro like hot cakes since I showed them in Vegas..
You can change your panels in under 5 minutes, and your facade will outlast anything else on the market.
That, and it's cleeeeean!